1. Field of the Invention
The invention relates to a process for operating a calender with a roll stack that features two end rolls and several middle rolls arranged therebetween. The rolls rest against one another in a press direction, with at least one roll featuring an elastic surface.
2. Discussion of Background Information
Such calenders are used in particular to glaze paper or cardboard webs. The invention is described below on the basis of the treatment of a paper web. However, it can also be used in the same way with other material webs, with which similar problems occur.
During the glazing of a paper web, the paper web is guided through a calender and into nips that are formed between hard and soft rolls (a soft roll is a roll with an elastic surface). In this way, the paper web is acted upon by increased pressure and, if necessary, also by increased temperature. In the case of calenders of more recent construction types, e.g., the “Janus calenders,” rolls are used which are covered with a plastic coating. It has been observed that in many cases crosswise stripes occur on the paper web after a certain working time. As soon as these stripes become visible, the paper web becomes useless and forms waste. The reasons for this so-called barring formation have not yet been conclusively established. However, it is assumed that they are the effects of a vibration phenomenon. However, vibrations are virtually unavoidable in a calender.
In the barring formation, the elastic surface of the soft roll is changed. It has not yet been conclusively established precisely what this change comprises. The following possibilities are currently assumed: the roll develops a waviness on the surface, i.e., a hill and valley structure, that the roll becomes polygonal, or that the roll develops alternating zones of varying surface quality in the circumferential direction, e.g., varying roughness. Regardless of the particular type of change, after the barring formation, periodic stripes appear on the circumference of the roll running in the axial direction. As a result, corresponding stripes then appear on the paper web, whereby the paper web is to be considered as waste, i.e., the web is rejected material, by the time the stripes become visible at the latest.
When a barring occurs, the roll that causes the barring formation must be removed and reground or finished. The service life of such a soft roll is therefore significantly limited.
The barring phenomenon also occurs in other calender stacks, i.e., from calenders that are formed solely of hard rolls. However, the development of the barring on the paper web takes substantially longer. It is assumed that the barring formation is to be attributed to other causes, in particular interferences in the paper web.
In “Calender Vibration—A Simulation Study and a Cure,” Tappi Journal, Vol. 52, No. 7, July 1969, pages 1356 through 1361, it is proposed to embody a guide roll such that its position can be changed relative to the roll stack. This is intended to simulate a change in diameter of a roll.
The document “Calender barring on paper machines—practical conclusions and recommendations,” Tappi Journal, Vol. 58, No. 8, August 1975, pages 147 through 151, proposes a similar solution, whereby it is alternatively proposed to displace the rolls between 20 mm and 60 mm relative to the roll stack. However, this displacement has the disadvantage that it changes the geometry of the roll stack relatively drastically, which can have a negative impact on the line loads and the line load distributions in the individual nips.
The use of plastic-coated rolls has led to a new type of barring, in which patterns impress themselves on the surface of the soft rolls in a relatively short time.